Animal and preliminary human studies of adult stem cell therapy for myocardial repair have shown an overall improvement of cardiac function. However, repair of injured myocardium remains a serious challenge. Emerging evidence from in vitro and preclinical studies suggests that ischemic environment including inflammation and oxidative stress, have adverse effects on stem cell function. MicroRNAs (miRNA; 20-25 nucleotides), are known to regulate several cellular processes such as proliferation, differentiation, cell metabolism, apoptosis and angiogenesis. The premise of our proposed research is based on preliminary observations that 1) prolonged inflammation impairs tube formation ability of mouse BM-derived EPCs, in vitro 2) miRNA array data from EPCs, in response to inflammatory stimuli, indicates an up-regulation of number of miRNAs related to cell survival/death and angiogenesis with a robust increase in miR-377, 3) myocardial infarction (MI) increased miR-377 expression in the myocardium, circulating EPCs and plasma, 3d post-MI; 4) pre-miR-377 transfection in EPCs inhibits protein expression of STK35 (a novel kinase localized in the nucleus), a potential target gene of miR-377, 5) miR- 377-mimics reduced EPC migration and vascular tube formation 6) Furthermore, STK35-knockdown in EPCs reduced HDAC5 phosphorylation and increased GATA2 acetylation (indicating transcriptional repression) and was associated with reduced expression of cytokines and growth factors, and reduced migration and vascular tube formation. 7) Most interestingly, in mouse MI model, intramyocardial transplantation of ex vivo anti-miR-377- transfected EPCs decreased cardiomyocyte apoptosis and infarct size, enhanced neovascularization and LV function and therefore efficient myocardial repair. These data provide convincing evidence that anti-miR-377 therapy might enhance the EPC engraftment and function in myocardial ischemia leading to efficient myocardial repair. Our central hypothesis is that ex vivo miR-377 knockdown and intramyocardial transplantation of EPCs augments their function by stimulating STK35-dependent phosphorylation and nuclear export of HDAC5 leading to increased GATA2-mediated transcription of genes (including cytokines and growth factors) that promote neovascularization and cardiomyocyte viability, thus resulting in efficient myocardial repair after injury. The hypotheses will be tested under the following 3 specific aims: 1) Define the role of miR-377 in EPC biology and function, in vitro 2) Elucidate the molecular mechanisms by which miR-377 regulates STK35-mediated EPC function and 3) Determine the critical role of miR-377 in EPC-mediated myocardial repair, using a mouse model of myocardial infarction.